Reduced properties are also used to define the Peng–Robinson equation of state, a model designed to provide reasonable accuracy near the critical point.[2] They are also used to critical exponents, which describe the behaviour of physical quantities near continuous phase transitions.[3]

Reduced temperature

where the actual temperature and critical temperature are expressed in absolute temperature scales (either Kelvin or Rankine). Both the reduced temperature and the reduced pressure are often used in thermodynamical formulas like the Peng–Robinson equation of state.

Reduced specific volume

The reduced specific volume (or "pseudo-reduced specific volume") of a fluid is computed from the ideal gas law at the substance's critical pressure and temperature:[1]

This property is useful when the specific volume and either temperature or pressure are known, in which case the missing third property can be computed directly.